In connectionless Ethernet Networks, Spanning Tree Protocol (STP), and in particular Rapid Spanning Tree Protocol (RSTP), is used to determine a loop-free active infrastructure in which redundant network resources are present. An example of network topology is the ring topology, where RSTP determines an alternative link when forwarding of user data is stopped. STP and RSTP can be implemented in other network topologies, for example the meshed and hierarchical network topology types.
A specific behaviour of STP, and in particular of RSTP, is that when the active topology changes (generally because of an Ethernet link status is increasing or decreasing) a sequence of notifications reaches all of the nodes, generating flushes in the Forwarding Database (FDB) of almost all the interfaces of each node. Here, flushing involves removing from the FDBs some dynamic entries, each of them correlating a Media Access Control (MAC) address to a bridge port of a bridge (i.e. a node), a relationship established by a learning procedure of each bridge.
This behaviour is undesirable because it causes unwanted flushing and subsequent flooding in many of the nodes of the network. Flooding involves broadcasting a frame to all the bridge ports that are in the member set of a particular Virtual Local Area Network (VLAN). This potentially generates a large amount of traffic using bandwidth in the network. This is particularly undesirable when the topology implies changes of path for only a reduced zone of the network (near to the node which notifies the topology change).